03b: Thyroid Flashcards
Major secretory product of thyroid gland is (X).
X = T4 (thyroxine)
(T4/T3/rT3) are active forms. Which of these are produced exclusively in thyroid gland?
T3;
T4 (T3 and rT3 can be produced from T4 in peripheral tissues)
(X) enzymes in peripheral tissues catalyze production of (T3/rT3/T4) from (T3/rT3/T4).
X = 5’-deiodinases;
T3 or rT3;
T4
T/F: Most circulating T3 is formed from degradation of T4, not from biosynthesis.
True
T4 biosynthesis: two coupled (X) rings form (Y).
X = Tyr Y = thyronine
TRH is formed in (X) and carried to (Y) cells to induce TSH secretion. Is TRH a clinically useful measurement of thyroid function?
X = PVN of hypothalamus Y = thyrotropes (of ant pit)
No - conc too low in peripheral circulation
Most important regulator of TRH production:
Long-loop negative feedback by T3/T4
List the four hormones, released from (X), that have same alpha and different beta subunits.
X = anterior pituitary;
LH, FSH, TSH, hCG
Most negative feedback control regulating Thyroid hormone levels takes place between (thyroid/pituitary) and (pituitary/hypothalamus).
Thyroid and pituitary
Within the thyrotrope, (T3/rT3/T4) is the hormone that produces negative feedback effects. Most of it comes from (X).
T3;
X = intra-pituitary conversion of T4 to T3
(T3/T4) negative feedback within thyrotrope has which mechanism?
T3;
Binds nuclear receptors and inhibits expression of both TSH subunits
List the levels at which glucocorticoids (stimulate/inhibit) thyroid axis.
Inhibit;
- TRH synthesis
- Thyrotrope responsiveness to TRH
- Deiodinase activity
Under intense TSH stimulation, you’d expect the thyroid follicle to appear:
Hyperplastic (shrunken); colloid undergoing resorption for hormone synthesis
Thyroid hormone biosynthesis requires which two components?
- Thyroglobulin
2. Iodide
Iodide enters thyroid follicular cell via (X). This is a(n) (uphill/downhill) movement.
X = Na/I symporter (NIS)
Uphill (very high iodide conc in cell compared to out)
Which transport proteins/pumps are present on (apical/basal) membrane of thyroid follicular cell to allow iodide entry?
Basal;
- NIS (symporter)
- Na/K Pump (too keep Na low in cell)
T/F: Iodide entry into follicular cell is rate-limiting step of thyroid hormone production.
True
TPO (thyroid peroxidase) is key enzyme that catalyzes which steps in thyroid hormone synthesis?
- Oxidization of iodide (to iodine)
- Incorporation into thyroglobulin
- Coupling (ex: T2 and T2 to form T4)
Immediately (before/after) exiting (apical/basal) membrane of follicular cell, iodide is (oxidized/reduced) to (X).
After; apical;
Oxidized
X = iodine
(by TPO enzyme)
The iodide attaches to (X) residues of thyroglobulin.
X = Tyr
Thyroid hormone synthesis: The actions of TPO are mediated by (X), which is generated by (Y).
X = H2O2 Y = NADPH oxidase
Iodide transport across apical membrane of follicular cell is via (X) channel/ATPase/transporter.
X = Pendrin (I/Cl antiporter)
Tyrosine organification/iodination yields which products?
- MIT (T1) or
2. DIT (T2)
Thyroid colloid stores enough organified thyroglobulin for (X) period of time.
X = 2-3 months
T/F: When thyroid hormone is needed, biosynthesis of thyroglobulin starts de novo and stored iodide (in colloid) is available.
False - colloid stores months’ worth of already organified thyroglobulin
Upon stimulation of thyroid gland, follicular cells endocytose (X), which fuse with (Y). Finally, (Z) process forms mature thyroid hormones.
X = colloid Y = intracellular lysosomes Z = proteolysis of thyroglobulins
Thyroid hormone synthesis: following proteolysis in lysosome, (X) enter circulation. What’s the fate of the byproducts?
X = mature hormones (T3/T4)
MIT, DIT stay in follicular cell and deiodinated (by thyroid deiodinase); recycling
Most of the iodide used in thyroid hormone synthesis comes from:
Recycling of MIT, DIT iodides following their deiodination
Perchlorate, a competitive inhibitor of NIS transporter, can be administered clinically to treat:
Hyperthyroidism (since it slows down thyroid hormone production)
The Wolff-Chaikoff effect is essentially (stimulation/inhibition) of (X) in response to (Y).
Inhibition;
X = TPO
Y = high circulating iodide
The Wolff-Chaikoff effect typically lasts how long? What’s the mechanism that causes escape from this effect?
A few days;
Down-regulation of NIS allows thyroid to adjust to high iodide levels (and TPO inhibition is released)
About (X) T4 and (Y) T3 are secreted daily.
X = 80-100 micrograms Y = 5 micrograms
Majority of T4/T3 in plasma is (free/bound).
Bound to TBG, albumin, or transthyretin
The metabolic rate of many tissues is determined by the (free/bound) fraction of T3/T4.
Free
T/F: Physiological thyroid status best correlates with T3/T4 that’s bound to TBG, not albumin.
False - free hormone fraction
T/F: Thyroid hormones are transported into target cells via passive diffusion.
False - via membrane carriers
T/F: Rate-limiting step of thyroid hormone action is transport into target cell.
True
Once inside target cell, thyroid hormone binds (X) receptors, in conjunction with (Y).
X = non-histone protein nuclear receptors Y = TRE (thyroid hormone response elements)
List some proteins with genes that are regulated by T3.
- T3’s own receptor (and TRE)
- TRH, TSH
- GH
- Myosin heavy chain
Certain tissues, such as (X), with (many/few) (Y), have significant response to T3 and large changes in protein synthesis/enzyme patterns.
X = liver
Many;
Y = nuclear receptors for T3
List the two key ways that myocardial tissues respond to T3.
- Increasing adrenergic receptors
2. Increasing actomyosin ATPase activity
T/F: Only the active thyroid hormone (T3) enters the target cell.
False - T4 enters too (encounters intracellular deiodinases that convert it to T3)
T/F: Only the active thyroid hormone (T3) enters the target cell nucleus.
True
T/F: Intracellular deiodinases (in target cell) only work on T4 (to produce T3).
False - also cleave T3 to T2/rT3 or T4 to rT3
T3 effect: (increase/decrease) bone formation/maturation.
Increase
T3 effect: (increase/decrease) oxygen consumption, BMR, and heat production.
Increase
T3 effect: (increase/decrease) CO and ventilation. (Up/down)-regulation of (X) adrenergic receptors.
Increase;
Up-regulation;
X = beta
T3 effect: increase in which processes in skeletal muscle?
Glycogenolysis and protein catabolism
T3 effect: increase cholesterol (synthesis/degradation) and (increase/decrease) LDL clearance/receptors.
Synthesis;
Increase
T/F: Thyroid hormone normally stimulates exclusively catabolic pathways.
False - both anabolic and catabolic pathways (supports balance)
T3 effect: there’s a(n) (increase/decrease) in need for fuel. This is met by (increase/decrease) in which factors?
Increase; increase
- GI motility/absorption
- Increase appetite
T/F: Like sympathetic stimulation, TPR increases with increase in T3.
False - T3 increases production of vasodilatory metabolites and TPR falls
T/F: Normal whole body growth cannot occur without adequate T3 levels, even if GH is normal.
True
A hypothyroid child will have a bone age that’s (more/less) than chronological age.
Less
Untreated (hyper/hypo)-thyroidism in children can lead to which brain impairment?
Hypo;
Severe, irreversible mental retardation
Fetal thyroid gland forms (early/late) in (X) trimester. By (Y) stage, the fetal hypo-pit-thyroid axis is fully operational.
Early;
X = first
Y = mid-gestation
During development, fetus receives (T3/T4/TSH) from mother/placenta to supply its needs. Thus, maternal (X) levels increase to meet greater demands.
T3 and T4;
Iodide and thyroid hormone
The (increase/decrease) in thyroid hormone output during pregnancy is not met with (hyper/hypo)-thyroidism. Why?
Increase; hyper
Increase in TBG (due to estrogen) to bind the increased hormone levels
Early in pregnancy, it’s actually (X) hormone that stimulates thyroid to secrete.
X = hCG (not TSH)
First trimester of pregnancy: (increase/decrease) hCG, (increase/decrease) TSH, (increase/decrease) thyroid hormones.
Increase;
Decrease (neg feedback from increased T3/T4);
Increase (continued secretion stimulated by hCG)
Grave’s disease is an example of (primary/secondary) (X).
Primary;
X = Hyperthyroidism
Exophthalmos is seen in (X) condition. What’s the reason for this?
X = hyperthyroidism;
Inflammation/swelling in orbital tissues (pushes eyeballs forward)
Hashimoto’s disease is (primary/secondary) (X). What’s the central issue in this disease?
Primary;
X = hypothyroidism
Anti-thyroid Ab (autoimmune); leads to eventual destruction of thyroid gland
What’s the mechanism behind swelling of (X) gland in Hashimoto’s disease?
X = thyroid
Inflitration of lymphocytes as well as fibrosis
Silent, post-partum (X) condition begins with (hyper/hypo)-thyroidism, then (hyper/hypo)-thydroidism until normal function is reached again.
X = thyroiditis (inflammation of thyroid gland)
Hyper; hypo
It’s a “rebound” or (increase/decrease) in (X) function that causes post-partum thyroiditis.
Increase;
X = immune system